The use of reflected Placido rings to analyze a subject's eye is known. For instance, in U.S. Pat. No. 3,248,162 (Apr. 26, 1966) to Knoll et al., various methods of positioning and illuminating a plurality of reflecting rings in front of a subject's cornea are identified. Such methods include photokeratoscopes wherein the rings are disposed on a plane surface which is held in front of the subject's cornea and illuminated by a light source and to one side of the cornea, reflecting rings formed on the interior surface of a sphere with the light source outside of the sphere, reflecting rings formed on the inner surface of a cylinder with the light source disposed outside of the cylinder, or reflecting rings disposed on the interior surface of a cylindrical cage with the light source disposed within the cage itself.
Illuminating the rings causes reflections of the rings to appear on the cornea, and these reflections are then depicted (e.g. either optically or by photography) and then analyzed by an eye specialist. The reflecting rings are circular, and deviations of the cornea from sphericity cause bumps or indentations indicative of irregularities in the cornea to be present in the reflections of the rings. Unequally spaced rings can be indicative of regular deviations from sphericity, and elliptical reflections may indicate a toric cornea.
Other devices which use reflecting patterns to analyze a subject's eye are described in U.S. Pat. No. 1,750,931 to Kellner et al. (Mar. 18, 1930), U.S. Pat. No. 5,018,850 to Gersten et al. (May 28, 1991), U.S. Pat. No. 4,772,115 to Gersten et al. (Sep. 20, 1988), U.S. Pat. No. 3,932,030 to Hasegawa et al. (Jan. 13, 1976), U.S. Pat. No. 5,194,882 to Penney (Mar. 16, 1993); U.S. Pat. No. 4,685,140 to Mount, II (Aug. 4, 1987); U.S. Pat. No. 5,227,818 to El Hage (Jul. 13, 1993); U.S. Pat. No. 5,416,539 to Gersten et al. (May 16, 1995); U.S. Pat. No. 5,418,582 to van Saarloos (May 23, 1995); and U.S. Pat. No. 5,526,072 to El Hage (Jun. 11, 1996).
Exemplary of these devices is U.S. Pat. No. 3,797,921 to Kilmer et al. (Mar. 19, 1974), the contents of which are incorporated by this reference. Kilmer et al. discloses a photographic apparatus for determining the corneal radius. The apparatus uses a transparent, concave screen having one or more opaque, concentric rings placed thereon. Light passes through the screen to the subject's eye, causing the concentric ring pattern to be reflected on the subject's eye. A photograph is taken and analyzed, and the corneal radius determined.
Although the aforementioned devices are usually adequate for most purposes, sometimes it is desirable to change the image or pattern illuminated by the apparatus (e.g. from a Placido ring pattern to a checkerboard pattern). Unfortunately, with the axisymmetrical shape of the Placido screens to date (e.g., on cylindrical and dish-shaped screens), changing the pattern has proven difficult. Planar screens, although potentially easier to change the pattern thereon, suffer from an inadequate reflective pattern for the amount of space used. It would thus be an improvement in the art to have an easy to manufacture non-planar screen which could be readily associated and disassociated with chosen patterns.